1. Field of the Invention
The present invention generally relates to an operating method of an electrically erasable programmable read-only memory (EEPROM), in particular, to an operating method of a virtual-ground NOR memory.
2. Description of Related Art
Electrically erasable programmable read-only memory (EEPROM) is a non-volatile memory that is able to write, read or erase data for many times and the written data can be still stored even the power is broken. Flash memory is the most typical EEPROM. By replacing the doped polysilicon floating gate of a flash memory with silicon nitride, a charge-trapping memory is obtained as another EEPROM. In a charge-trapping memory, the charge-trapping layer is usually made of silicon nitride, a layer of silicon oxide is usually disposed respectively on and under the charge-trapping layer, and the control gate and the substrate are usually made of polysilicon or silicon. Accordingly, such a device is usually referred as a silicon/oxide/nitride/oxide/silicon (SONOS) device. Due to the charge-trapping characteristic of silicon nitride, electrons injected into a charge-trapping layer are usually localized in certain regions of the charge-trapping layer. Generally, a region close to the drain and a region close to the source in a charge-trapping layer can respectively store a bit. Thereby, each memory cell in a charge-trapping memory can store two bits.
When a SONOS memory is programmed, first, a single memory cell is selected. A word line corresponding to the control gate of the selected memory cell is applied with a bias, a bit line corresponding to the drain of the selected memory cell is also applied with a bias, the other word lines are applied with a voltage of 0V, and the other bit lines are applied with a voltage of 0V or are floated. As a result, electrons or holes are injected into a region close to the drain in the charge-trapping layer and the threshold voltage of the memory cell is changed. However, the foregoing method for programming a SONOS memory one by one is very time consuming, and reading error may be caused because the memory has a large threshold voltage distribution.